The present invention relates generally to tool steels for hot and cold work applications and, more particularly, to an improved chromium hot work die steel of the AISI/SAE type H-13 possessing high impact toughness and thermal fatigue resistance. The improved alloy of the present invention is particularly suitable for use as a die steel in aluminum die casting, for example. Other hot work applications for the improved steel of the present invention include extrusion dies for aluminum and magnesium, dummy blocks and mandrels for brass and aluminum extrusion, hot press dies, punches and dies for shell piercing, die inserts for forging dies, hot shear blades to mention a few. Cold work applications for the present invention include cold heading dies, intermediate rolls for use on a Sendzimir mill and the like.
Heretofore, a commonly used hot work steel for such applications has been standard type H-13 having the following chemistry, in weight %, as published in the Metals Handbook, Ninth Ed., Volume 3, page 422: 0.32-0.45 C; 0.20-0.50 Mn; 0.080-1.20 Si; 4.75-5.50 Cr; 0.30 max Ni; 1.10-1.75 Mo and; 0.80-1.20 V. While H-13 steel has found extensive use as a hot die material due to its elevated temperature hardness and strength properties, it does fall victim to a shortened service life caused by brittle failure and/or thermal fatigue. The dies used in pressure die casting of aluminum, for example, are subject to thermal cycling resulting from constant heating and cooling during operation at temperatures up to about 900.degree. to 1,000.degree. F. It is sometimes necessary to slow the production rate in order to prevent overheating of conventional die steel material such as H-13. The casting dies are also subject to significant loading during use at such elevated temperatures, which provides an ideal condition for brittle failure to take place. In addition, the vanadium alloy constituent in conventional H-13 steel is rather costly and subject, in recent times, to wide price fluctuations, making vanadium a significant factor in the cost of H-13 steel.
Prior attempts have been made to partially or fully substitute niobium for vanadium in H-13 to control grain size, but these have not proven successful in large production heats due to the formation of large niobium carbide particles which are harmful to high temperature properties and grain size.
It is, therefore, an object of the present invention to provide an improved H-13 type steel with greatly improved impact toughness and resistance to thermal fatigue or heat check resistance while maintaining comparable elevated temperature hardness and strength characteristics of conventional and premium grade H-13 steels. In addition, the present invention exhibits such improved properties while significantly reducing the amount of the expensive vanadium alloy previously required in H-13 steel. A process according to the present invention provides a more homogeneous microstructure which still further increases the toughness as well as the heat check resistance of the material. The resulting hot work die steel of the invention provides in die casting operations increased production rates as well as increased tool life over conventional die steels.